Code snippet showing examples of what's possible with first class functions

first_class_functions.py

from typing import Callable

def double(n: int) -> int:
    return 2 * n

def is_even(n: int) -> bool:
    return n % 2 == 0

def make_adder(n: int) -> Callable[int, int]:

    def adder(m: int) -> int:
        return n + m

    return adder

if __name__ == "__main__":
    # 1. Functions can be assigned to variables
    times2 = double
    print(times2(21)) # 42

    # 2. Functions can be passed to other functions arguments
    evens_under10 = list(filter(is_even, range(10)))
    print(evens_under10) # [0, 2, 4, 6, 8]

    # 3. Functions can be returned from functions
    add10 = make_adder(10)
    print(add10(5)) # 15